Thermo-sensitive recording processes have been hitherto used as an image-formation process wherein a thermo-sensitive color-developing sheet is selectively heated and color developed by heating means, such as a thermal head, according to image information. The thermo-sensitive color developing sheet used in these processes includes a structure having a base sheet and a layer of a colorless or light colored leuco dye at normal temperatures and a color developer. On heating, the leuco dye and the color developer react with each other, thereby forming a colored image.
However, problems are involved in that the colored image formed on the thermo-sensitive color-developing disappears on storage over a long time and that the background or image-free portions of the thermo-sensitive color-developing sheet suffer color development. In addition, there is another problem that it is very difficult to obtain color images whose gradation is continuous.
In recent years, hot melt-type thermo-sensitive transfer recording material has been utilized wherein a thermally fusible ink layer dispersing a pigment or dye is formed on a base sheet. In addition, sublimation-type thermo-sensitive transfer recording materials have been used which have, on a base sheet, a thermo-sensitive, sublimable ink layer having a thermally diffusible dye dispersed in binders. Generally, a thermo-sensitive transfer recording process is used wherein these recording materials are selectively heated in accordance with image information by heating means, such as a thermal head, for transferring the ink from the thermo-sensitive transfer recording material to a sheet for receiving the ink such as a recording paper, thereby forming an image. According to this thermo-sensitive transfer recording process, multi-color images can be formed. If a sublimation-type thermo-sensitive transfer recording material is used, there can be obtained images with continuous gradation. Such images can be readily formed from television signals.
The base sheets for the thermo-sensitive transfer recording material which have been heretofore used include condenser paper, cellophane sheets, cellulose acetate films, and plastic films such as polyester films, polypropylene films and the like, generally with a thickness of approximately 10.mu.m. Of these base sheets, condenser paper is better in view of economic considerations. However, plastic films have been widely used in view of their high resistance to breakage at the time of coating of the ink layer, uniform thickness, surface smoothness and handling properties within printers. Preferably, a polyester film has been used since high strength is obtained irrespective of the film thickness.
However, when thermo-sensitive transfer recording is effected using a thermo-sensitive transfer recording material which makes use of a polyester film as a base sheet, sticking often takes place wherein the base sheet is stuck on the thermal head. This is a disadvantage since it is very difficult to run the recording material and the recording material may break.
In order to solve this problem, various attempts have been made to provide a heat-resistant lubricating layer on a side of the base sheet opposite to the ink layer.
Although these heat-resistant lubricating layers will substantially reduce sticking during the course of thermal fusion-type, thermo-sensitive transfer recording, such sticking in sublimation-type, thermo-sensitive transfer recording cannot be reduced to a satisfactory extent. This is because with sublimation-type, thermo-sensitive transfer recording, the thermal energy required is approximately 1.5 times higher than that required in the thermal fusion-type transfer recording at the time of the thermal transfer.
In order to substantially reduce sticking to a satisfactory extent during sublimation-type thermo-sensitive transfer recording, extensive studies have been made on the types of heat-resistant resins and lubricating materials contained in the resins. For instance, there have been proposed formation, on a base sheet, of a layer which is formed of a reaction product of an acryl polyol resin and an isocyanate compound to which phosphoric esters are added and formation of a layer which is made of a reaction product of a polyvinyl butyral and an isocyanate compound, to which alkali metal phosphates or alkaline earth metal salts and fillers such as calcium carbonate are added.
The heat-resistant lubricating layer to which phosphates are added is effective in imparting lubricity to the layer. However, since the phosphoric esters are formulated as an additive, the ester is liable to separate from the resin, thus dispersing the ester therein during use. This contaminates a thermal head during the course of the transfer which substantially inhibits the transfer process and causes other associated problems to arise. In addition, the phosphoric esters are undesirably migrated to the ink layer during storage of the recording material, with the attendant problem that the concentration and color tone of a dye in the ink layer are changed.